Could Norwegian Engineers Really Build A Floating Tunnel In A Fjord?

For those of us who live in busy, built-up cities, travelling through tunnels is an everyday occurrence. But engineers in Norway want to build a world-first in one of their fjords – an underwater, tube-shaped tunnel that floats. This is being proposed as part of a huge infrastructure project in Norway – an upgrade to highway E39, which runs from Kristiansand in the south to Trondheim in the north, a route which is 1100 km long. But thanks to Norway’s (staggeringly beautiful, but occasionally inconvenient) geology, it cuts across seven fjords. Which means that your ‘drive’ between the two cities actually includes seven ferry trips, and takes 21 hours. The upgrade aims to make the route entirely ferry-free, to reduce travel time to just 11 hours. And this is where the proposal for a floaty bridge-tunnel comes in.

Now, as you know, I’m a big fan of both bridges and tunnels, but a structure that seems to combine the two falls outside of my comfort zone, so I spoke to Arianna Minoretti, a senior engineer at Statens Vegvesen – Norway’s public roads administration – to learn more. Along with her colleagues and a range of consultants, Arianna has been preparing the detailed feasibility study for one of the largest fjords on the route – Bjørnafjord. “For the user, the structure will be indistinguishable from a tube-shaped tunnel,” she told me, “but in terms of its behavior, it has much more in common with a bridge.” This description might go some way to explaining the dizzying array of names given to this structure – officially called a submerged floating tube-bridge (SFTB), it’s also known suspended tunnel or, most tellingly, an Archimedes Bridge.

How comfortable would you feel driving through a submerged tunnel? (Image credit: Statens Vegvesen / NPRA)

You probably know at least two things about the Archimedes principle: (1) that it is all about how stuff floats, and (2) that the great Greek mathematician, after whom it is named, first came up with the idea while sitting in the bath. He noticed that the level of water in his bathtub rose as he got in, and realized that this could be used to determine the volume of a body (his, in this case!) immersed in water. That led to the idea of buoyancy, which, in short, says that as long as an object weighs less than the weight of the water it displaces, it will float. And this is the key concept behind a submerged floating tube-bridge (SFTB) – you want it to float above most of the water while still remaining submerged.

In Arianna’s vision, Bjørnafjord’s SFTB would be made from two long concrete tubes – one for each direction of travel – that are bound to each other via connecting trusses. Sections of these tubes would be built in a dry dock before being sealed and floated to where they will be installed. Once the sections are joined to form two continuous tubes, ballast (in the form of water) is allowed to enter the tubes, adjusting their mass and letting them sink to the correct level. Once there, the SFTB can be either tethered to the bedrock below, or attached to floating pontoons, after being joined to both shorelines. When in use, these tethers and/or pontoons will hold the tube-bridge in a fixed, highly-stable position, so there’s no chance you’ll feel seasick while driving through the tunnel!

This particular SFTB would sit 30 meters (about 100 feet) below the surface of the water. Marine traffic regularly enters Bjørnafjord, so the depth has been chosen with that in mind – even the largest ships common to these waters could pass over the tube-bridge. And it wouldn’t be an obstacle for the submarines that use it the fjord either – 30 meters is more than enough water for them to travel through… as is the 500 meters-or-so (about 1,640 feet) of water that would be underneath the structure. It really is a big fjord. As you might expect, building an SFTB comes with lots of engineering challenges. While too deep to be affected by the weather, there is a lot to consider, said Arianna. “Knowledge of the environment is very important, so in addition to our detailed simulation, we have started a campaign of measurement of wind speed, current, bedrock geology etc., so that we can accurately compare our model to the real-world environmental loads.”

This project has brought together experts from lots of different sectors – from tunnel engineers and bridge builders to sub-sea mariners and ecologists. According to Arianna, the success of this proposal could kick-start a new field of construction that will have implications way beyond Norway. But at this moment, this is only a proposal – one of three being assessed for crossing this particular fjord – the other two are a suspension bridge and a floating bridge (like this one near Seattle).

But in this environment, even a bridge could be a challenge – for a start, it’d need to span a distance of close to 5 km (3 miles), which would make it one of the longest in the world. And it would need to withstand harsh winds and corrosive sea air – neither of which would be an issue for the tube-bridge. Arianna’s preference is clear, “I really hope we get to build the SFTB. For an engineer like me it’s a dream project – it would be the first in the world, and it answers the question of how best to pass through incredible landscapes that need to be preserved, without damaging them”.

A final decision on Bjørnafjord’s future is expected by the end of 2016. I must admit, I’m with Arianna and Archimedes on this one… EUREKA!